Cements are materials which set and harden after combining with water, as a result of chemical reactions with the mixing water and, after hardening, retain strength and stability even upon exposure to water. Most construction cements today are hydraulic cements, and most of these are based upon Portland cement, which is made primarily from limestone, certain clay minerals, and gypsum, in a high temperature process that drives off carbon dioxide and chemically combines the primary ingredients into new compounds.
Setting and hardening of hydraulic cements is caused by the formation of water-containing compounds, forming as a result of reactions between cement components and water. The reaction and the reaction products are referred to as hydration and hydrates or hydrate phases, respectively. As a result of the immediately starting reactions, a stiffening can be observed which is very small in the beginning, but which increases with time. After reaching a certain level, this point in time is referred to as the start of setting. The consecutive further consolidation is called setting, after which the phase of hardening begins. The compressive strength of the material then grows steadily, over a period which ranges from a few days in the case of “ultra-rapid-hardening” cements, to several years in the case of ordinary cements.
The compressive strength of a cement is an important characteristic, especially in applications that place the cement under stressful conditions, such as those that exist when the cement is used as a casing in a wellbore in a subterranean formation. Cements are generally dense compositions, but in some applications it may be desirable to have the compressive strength of a typical cement coupled with a light weight. Currently there is a paucity of such compositions available that have both these traits. Thus, there is a continuing need for the development of cement blend compositions.